The present invention relates to a flexible pipe that can be used for transporting fluids, such as hydrocarbons for example.
Several types of flexible pipe are used. Some flexible pipes comprise, from the inside outward, an internal sealing sheath made of a plastic, an elastomer or another relatively flexible suitable material; an unsealed flexible metal tube that has to withstand the forces developed by the pressure of the fluid flowing in the pipe; one or more armor plies and at least one external sealing sheath made of a polymeric material. This type of flexible pipe is often called a smooth-bore pipe by experts in the field.
Other flexible pipes, called rough-bore pipes, comprise, from the inside outward, an unsealed flexible metal tube, called a carcass, formed by a profile wound in turns and mutually interlocked, such as, for example, an interlocked strip or an interlocked shaped wire such as a T-shaped, U-shaped, S-shaped or zeta-shaped wire; an internal sealing sheath made of a polymeric material; one or more armor plies capable of withstanding the forces developed by the pressure of the fluid flowing in the pipe and the external forces to which the flexible pipe is subjected; and at least one external protected sheath of the polymeric type.
In the latter type of flexible pipe, the internal sealing sheath is extruded, continuously, directly over said carcass, which has interstices or gaps between the wound turns.
To ensure good contact between the internal sealing sheath and the metal carcass, it is necessary for the inside diameter of the internal sealing sheath to be as close as possible and even equal to the outside diameter of the flexible metal carcass.
During manufacture of a rough-bore flexible pipe, the internal sealing sheath, which is extruded over the metal carcass, contracts onto the latter during cooling. Depending on the materials used for producing the internal sealing sheath, after cooling deformations called “shrinkage cavities” are observed, these cavities appearing on the internal face of said internal sealing sheath and especially on either side of the gaps between the turns of the metal carcass. Such shrinkage cavities are due, it would seem, to the differential shrinkage of the material used for the internal sealing sheath, because of the variation in the cooling gradient through the thickness of the internal sealing sheath, combined with the effect of the gaps between the turns of the metal carcass. Since the extruded plastic sealing sheath is in contact by its internal face with the metal carcass, which is at room temperature, this results in said internal face cooling very rapidly, thereby causing surface irregularities or shrinkage cavities; this phenomenon is exacerbated at the gaps between the turns of the metal carcass, the differential shrinkage at these points causing local variations in the thickness of the internal sealing sheath. When the sealing sheath is made of a semicrystalline polymer sensitive to the presence of surface defects causing a weakening of the sheath, possibly to the point of failure, such as PVDF (polyvinylidine fluoride) for example, this very often leads, in operation, to degradation (failure) of said sealing sheath, which then no longer fulfils its sealing function.
To remedy such a drawback and to solve the problem posed by the appearance of shrinkage cavities, a first solution consisted in placing, between the metal carcass and the internal sealing sheath, a thin sacrificial underlayer (thickness about 2 to 3 mm) made of a suitable material such as PVDF, which then serves as a heat shield. The internal sealing sheath is extruded over said sacrificial underlayer, but without any assurance that there is intimate bonding or “welding” between the sealing sheath and the sacrificial underlayer, so that cracks, that can propagate from the internal face of the underlayer to the outside, are blocked at the sealing sheath/sacrificial underlayer interface.
The major drawback of this solution is the slip that is liable to occur between the internal sealing sheath and the sacrificial underlayer at the ends of the flexible pipe, and the additional raw material and conversion costs incurred by the presence of said sacrificial underlayer.
Provision could be made to extrude a thinner sacrificial sheath (thickness less than or equal to 1 mm), but, because of the diameter of the extruded tube (greater than 10 cm), it is impossible for so thin a tube to be extruded on an industrial scale. It is therefore limited to a 2 to 3 mm thick sheath. In addition, the operation requires the intermediate sheath to be wound on an intermediate reel and, since the intermediate sheath is thin, it will buckle during winding.
To avoid these drawbacks, document FR 2 752 904 (COFLEXIP) proposed a process for manufacturing flexible pipes that consists in heating the flexible metal tube or metal carcass to a temperature of below 100° C., upstream of the extrusion means, so as in this way to avoid suddenly cooling the internal face during extrusion over the metal carcass.
For plastics of very low viscosity, it is necessary to heat the product to a very high temperature, and consequently to heat the carcass to a high temperature. This high temperature induces very substantial thermal creep in the gaps of the carcass, requiring the insertion, into these gaps, of a rod that limits the volume of creep in order to prevent blockage of the carcass. Such a rod is described in document FR 2 779 797 (COFLEXIP), but the spiraling of the rod in the gaps of the carcass is not simple to implement. Documents EP 0 749 546 (COFLEXIP-ELF ATOCHEM) and FR 2 732 441 (COFLEXIP) disclose the short-pitch helical winding of an intermediate strip for following and partially filling the gaps, and are therefore similar to the previous solution.
Another solution, proposed in document EP 166 385 (FURUKAWA), consists in winding, around the carcass, several layers of thin plastic (for example polyester) tapes (thickness about 0.5 mm for a tube about 2 to 8 cm inside diameter). This interlayer masks the gaps and prevents the sealing sheath from creeping into the gaps in the carcass. The details of the winding are not explained in the document.
The interlayer, by preventing the sealing sheath from creeping into the carcass, consequently also prevents the sheath from bonding to the carcass and therefore creates problems of slip between the two layers. For riser applications, this may cause the flexible pipe to deteriorate: since the gap between the turns is not controlled by the creep indentations, the carcass can slip under its own weight, the gaps between turns being canceled out and accumulating at the base of the riser, causing destruction of the carcass in the upper part.